Late blight on potatoes

Large leaf spots (lesions) are common with late blight. These were the main symptom seen in a commercial field on LI in 2009 when this disease was first found. The white growth on the leaf underside (see arrows, image below) contains hundreds of spores of the pathogen that could be dispersed by wind to other leaves in this field or beyond.

Upper leaf surface with late blight lesions (below)

Spores of the pathogen are visible on the lower (under) surface of leaves (below)

Close up of spores (below)

Leaf lesions can be small when they first start to develop (below left).

Symptoms also occur on other parts of potato plants including stems, petioles, and flowers. Following two photographs were taken on LI in 2006. White growth contains a lot of spores.

When first symptoms are mostly on stems (next two pictures below), it can be difficult to detect the onset of late blight, especially in a large planting as on a farm. All plant tissue above stem lesions will die shortly because the stem is completely affected, which is one reason that late blight is so devastating.